A female aquaintance is having problems with her toilet not flushing properly. The siphon has been replaced twice in recent months by a local plumber but when flushed the cistern only half empties. The cistern is marked as having a 7.5ltr capacity. The top of the siphon is level with the mark on the cistern. It is approx 8inches high. Would a taller siphon give a better flush? TIA